In rapid thermal processing of semiconductor wafers, a radiant source, such as a tungsten-halogen lamp or array of lamps, is commonly used to heat the wafer. A transparent window, typically made of quartz or sapphire, is used to separate the processing chamber from the heater. The window must be strong enough to withstand the pressure difference between the vacuum of the chamber and atmospheric pressure outside it, and therefore must be fairly thick. Quartz, however, is opaque to wavelengths above 4 .mu.m, and also absorbs a small amount of radiation in the region between 0.2 to 4 .mu.m. The total absorption grows with window thickness.
Under these conditions, in the absence of effective cooling, the window can reach high temperatures, typically up to 500.degree. C., after a few process cycles in the chamber. A thick, hot window can affect the process in a number of undesirable ways:
"First wafer effect"--The first few wafers in a batch are processed before the window reaches its equilibrium temperature, and the repeatability of the process may therefore be affected. PA1 Because of the low thermal conductivity of quartz, the temperature across the window may be non-uniform, thus affecting the uniformity of temperature across the wafer. PA1 Some rapid thermal chemical vapor deposition (RTCVD) processes, such as polysilicon deposition, can also cause deposition of material on hot parts of the vacuum chamber, including the window. The deposition reduces window transmittance, so that more lamp power must be used to reach the desired wafer temperature. Deposition on the window can also affect process uniformity and repeatability. PA1 Deposition on the window also increases the number of particles in the chamber, which can cause to contamination of the wafer, and leads to a requirement for more frequent cleaning. PA1 upper and lower transparent plates defining passages therebetween; and PA1 a cooling fluid flowing through the passages, which fluid is substantially transparent to the infrared radiation, such that the infrared radiation from the source passes through the plates and through the fluid in the passages to heat the object. PA1 an upper transparent plate, positioned adjacent to the radiation source and having a thickness sufficient so that the window assembly is able to withstand atmospheric pressure when the chamber is evacuated; PA1 a lower transparent plate, positioned adjacent to the object in the chamber and having a thickness substantially less than the thickness of the upper transparent plate; and PA1 supporting pieces fixed between the upper and lower plates so as to provide mechanical support to the lower plate and arranged so as to define passages between the plates through which a cooling fluid passes to cool the plates. PA1 a radiation source outside the chamber emitting radiation that includes infrared radiation; and PA1 a cooled window which seals the chamber and has passages therein through which a cooling fluid flows, which fluid is substantially transparent to the infrared radiation, so that the infrared radiation from the source passes through the fluid in the passages to heat an object in the chamber. PA1 sealing the chamber using a window having a passage therethrough; PA1 heating the object by means of radiation passing through the window, the radiation including infrared radiation; and PA1 cooling the window by flowing an infrared-transparent fluid through the passage, such that the infrared radiation passes through the fluid substantially without attenuation.
Methods and apparatus for window cooling are known in the semiconductor processing art. For example, U.S. Pat. No. 4,550,684, to Mahawili, which is incorporated herein by reference, describes a vapor deposition system that uses a lamp to heat a wafer with radiation in the wavelength range between 0.3 and 0.9 .mu.m. A window separates the lamp from a vacuum chamber containing the wafer. The window is constructed of two spaced-apart plates, between which water is pumped to control the window temperature. Although the water passing through the window absorbs infrared radiation emitted by the lamp, Mahawili considers this effect to be beneficial. He indicates that the desirable wavelength range for heating the wafer is between 0.3 and 0.9 .mu.m, and wavelengths outside this range should be excluded from the chamber.
U.S. Pat. No. 5,487,127, to Gronet et al., which is incorporated herein by reference, describes rapid thermal heating apparatus in which lamps are disposed in a plurality of light pipes in order to illuminate and supply heat to a semiconductor substrate. The light pipes are integrated with a liquid-cooled window, which separates the lamps from an evacuated processing chamber. The window is constructed so that cooling water can be injected into spaces between the light pipes without entering or passing in front of the light pipes themselves. The light pipes themselves are evacuated. The radiation from the lamps thus reaches the chamber without being attenuated by the water, but the integrated structure of lamps, evacuated light pipes and cooled window is complex and difficult to construct and maintain.